1. Field of the Invention
The present invention relates to a reinforced optical fiber having a bare fiber, such as an optical fiber with a fusion-spliced portion or an optical coupler, and a method of manufacture thereof.
2. Description of the Related Art
Methods of coupling optical fibers are well known in the art. In a conventional coupling method the coating at the coupling ends of two optical fibers is removed to expose bare fibers. The bare fibers butted against each other are then heated by means of laser light, electric discharge, etc. to fuse the bare fiber ends together.
"Recent Optical Fiber Coupler Technology", Optoelectronics (1988)No. 5, P.125 describes the fusion drawing method of coupling optical fiber cores. In the fusion drawing method the coating of a plurality optical fibers is removed to expose the bare optical fibers. The exposed optical fibers are bundled through heating fusion in which they are twisted or arranged in parallel. Subsequently the bundled portion is heated and drawn. Drawing is stopped when a characteristic such as coupling ratio reaches a predetermined value, thereby forming a coupling portion.
The conventional optical fiber core coupling methods leave unprotected a portion of bare optical fiber. In this unprotected state the bare optical fiber may become damaged from external factors such as external stress.
Japanese Patent Application Laid - Open No. 271208/1988 discloses a conventional reinforcing structure. A drawn portion (small-diameter molten portion) of the optical fiber core is suspended and nondrawn portions of (large-diameter molten portions) on both sides of the drawn portion of the optical fiber are fixed to a case or jig.
Japanese Patent Application Laid - No. 254406/1988 discloses a multi-core optical fiber coupler reinforcing structure. A pair of comb teeth-shaped fixing portions having a plurality of grooves are arranged on a substrate. The fixing portion are in a face-to-face relationship with each other in a longitudinal direction of the substrate. A plurality of optical fiber couplers are arranged in a direction perpendicular to a direction in which two optical fibers are arranged.
Typically these conventional reinforcing methods use a reinforcing member made of quartz glass having a coefficient of linear expansion equal to that of the optical fibers to prevent thermal stress from affecting the optical fibers. However, quartz glass is transparent and effects the optical properties of the optical fibers. Furthermore, quartz glass is a brittle material and easily damaged.
Additionally, typical conventional reinforcing methods cover the bare optical fiber portions with adhesive. This impairs the optical characteristics of the optical fibers.
Due to the difference in rigidity between the reinforcing member and the optical fiber the conventional reinforcing methods produce an undesirable bending stress which can concentrate on the optical fibers in the vicinity of the reinforcing member.
Furthermore, in conventional reinforcing methods the optical fibers are not hermetically sealed within the reinforcing structure. Water and other external environmental elements can enter the reinforcing structure. Thus the strength of transmission deteriorates, and variations in transmission loss occur at high temperatures or in a water environment.
In those conventional reinforcing methods using a reinforcing member made of a material whose coefficient of linear expansion differs from that of the optical fibers. Transmission loss changes due to a change in external environment temperature.